DNA polymerases replicate the genomes of living organisms. In addition to this central role in biology, DNA polymerases are also ubiquitous tools of biotechnology. They are widely used, e.g., for reverse transcription, amplification, labeling, and sequencing, all central technologies for a variety of applications, such as nucleic acid sequencing, nucleic acid amplification, cloning, protein engineering, diagnostics, molecular medicine, and many other technologies.
Because of their significance, DNA polymerases have been extensively studied, with a focus, e.g., on phylogenetic relationships among polymerases, structure of polymerases, structure-function features of polymerases, and the role of polymerases in DNA replication and other basic biological processes, as well as ways of using DNA polymerases in biotechnology. For a review of polymerases, see, e.g., Hubscher et al. (2002) “Eukaryotic DNA Polymerases” Annual Review of Biochemistry Vol. 71: 133-163, Alba (2001) “Protein Family Review: Replicative DNA Polymerases” Genome Biology 2(1): reviews 3002.1-3002.4, Steitz (1999) “DNA polymerases: structural diversity and common mechanisms” J Biol Chem 274:17395-17398, and Burgers et al. (2001) “Eukaryotic DNA polymerases: proposal for a revised nomenclature” J Biol. Chem. 276(47): 43487-90. Crystal structures have been solved for many polymerases, which often share a similar architecture. The basic mechanisms of action for many polymerases have been determined.
A fundamental application of DNA polymerases is in DNA sequencing technologies. From the classical Sanger sequencing method to recent “next-generation” sequencing (NGS) technologies, the nucleotide substrates used for sequencing have necessarily changed over time. The series of nucleotide modifications required by these rapidly changing technologies has introduced daunting tasks for DNA polymerase researchers to look for, design, or evolve compatible enzymes for ever-changing DNA sequencing chemistries. DNA polymerase mutants have been identified that have a variety of useful properties, including altered nucleotide analog incorporation abilities relative to wild-type counterpart enzymes. For example, VentA488L DNA polymerase can incorporate certain non-standard nucleotides with a higher efficiency than native Vent DNA polymerase. See Gardner et al. (2004) “Comparative Kinetics of Nucleotide Analog Incorporation by Vent DNA Polymerase” J. Biol. Chem. 279(12):11834-11842 and Gardner and Jack (1999) “Determinants of nucleotide sugar recognition in an archaeon DNA polymerase” Nucleic Acids Research 27(12):2545-2553. The altered residue in this mutant, A488, is predicted to be facing away from the nucleotide binding site of the enzyme. The pattern of relaxed specificity at this position roughly correlates with the size of the substituted amino acid side chain and affects incorporation by the enzyme of a variety of modified nucleotide sugars.
More recently, NGS technologies have introduced the need to adapt DNA polymerase enzymes to accept nucleotide substrates modified with reversible terminators on the 3′-OH, such as —ONH2. To this end, Chen and colleagues combined structural analyses with a “reconstructed evolutionary adaptive path” analysis to generate a TAQL616A variant that is able to efficiently incorporate both reversible and irreversible terminators. See Chen et al. (2010) “Reconstructed Evolutionary Adaptive Paths Give Polymerases Accepting Reversible Terminators for Sequencing and SNP Detection” Proc. Nat. Acad. Sci. 107(5):1948-1953. Modeling studies suggested that this variant might open space behind Phe-667, allowing it to accommodate the larger 3′ substituents. U.S. Pat. No. 8,999,676 to Emig et al. discloses additional modified polymerases that display improved properties useful for single molecule sequencing technologies based on fluorescent detection. In particular, substitution of φ29 DNA polymerase at positions E375 and K512 was found to enhance the ability of the polymerase to utilize non-natural, phosphate-labeled nucleotide analogs incorporating different fluorescent dyes.
Recently, Kokoris et al. have described a method, termed “sequencing by expansion” (SBX), that uses a DNA polymerase to transcribe the sequence of DNA onto a measurable polymer called an Xpandomer (see, e.g., U.S. Pat. No. 8,324,360 to Kokoris et al.). The transcribed sequence is encoded along the Xpandomer backbone in high signal-to-noise reporters that are separated by ˜10 nm and are designed for high signal-to-noise, well differentiated responses when read by nanopore-based sequencing systems. Xpandomers are generated from non-natural nucleotide analogs, termed XNTPs, characterized by bulky substituents that enable the Xpandomer backbone to be expanded following synthesis. Such XNTP analogs introduce novel challenges as substrates for currently available DNA polymerases.
Thus, new modified polymerases, e.g., modified polymerases that display improved properties useful for nanopore-based sequencing and other polymerase applications (e.g., DNA amplification, sequencing, labeling, detection, cloning, etc.), would find value in the art. The present invention provides new recombinant DNA polymerases with desirable properties, including the ability to incorporate nucleotide analogs with bulky substitutions with improved efficiency. Also provided are methods of making and using such polymerases, and many other features that will become apparent upon a complete review of the following.